1. Field of the Invention
The present invention relates to a scanning type probe microscope, and particularly to a scanning type probe microscope in which a deflection amount of a cantilever is detected by an optical lever method and a probe finely moves vertically to control a distance between the probe and a sample.
2. Description of Related Art
A scanning type probe microscope is a device that brings a probe with a sharp fore-end closer to a sample to measure a force produced between the probe and the sample, by which a shape of a sample surface and the like are measured at an atomic level.
The scanning type prove microscope includes a so-called cantilever with a very small spring constant. The cantilever has a probe at a fore-end thereof. A fore-end of the probe is extremely sharp in the nm order. A deflection amount of the cantilever by atomic force acting between the probe and the sample surface is measured, while horizontally moving the sample or the probe for scanning. A base portion of the cantilever is fixed to a cantilever holding portion.
As operation modes of the scanning type probe microscope, a contact mode and a dynamic mode are known. In the contact mode, a fine vertical movement amount z of the sample is adjusted so as to make the force between the sample and the probe constant in the scanning. Accordingly, during the scanning, a deflection amount θ of the cantilever is constant. The sample surface shape can be known from output of the fine vertical movement amount z of the sample when the deflection amount θ of the cantilever is adjusted to be constant.
In the dynamic mode, the sample surface is scanned while constantly vibrating the probe. When the probe approaches the sample at an atomic level, an amplitude and a resonance frequency of the vibration of the probe are affected by attracting force or repulsive force by the sample, thereby being changed. Consequently, the fine vertical movement amount z of the sample is adjusted so as to make the amplitude or the resonance frequency of the probe constant. The sample surface shape can be known from the output of the fine vertical movement amount z of the sample.
As means for measuring the deflection amount θ of the cantilever, a detection optical system by a method generally called optical lever method is used. In FIG. 4(a), one example of a detection optical system 40 by the conventional optical lever method is shown. A laser light source 43 and light detection means 44 are arranged in a plane including a long axis of a cantilever 42 above the cantilever 42 attached to a fore-end of a cantilever holding portion 41. The light detection means 44 is, for example, a quadrant photodiode.
Incident light 45 emitted from the laser light source 43 is reflected on an upper surface of the cantilever 42, so that reflected light 45a enters the light detection means 44. The light detection means 44 (the quadrant photodiode) has four light-receiving regions 44a. When the deflection amount θ of the cantilever 42 has a reference value, the reflected light 45a enters a center of the four light-receiving regions 44a. 
As shown in FIG. 4(b), when deflection occurs in the cantilever 42, thereby changing as shown by a cantilever 42a indicated by dashed lines, the reflected light 45a moves from the center of the light detection means 44 as shown by reflected light 45b indicated by a dashed line. The four light-receiving regions 44a output voltages in accordance with received light intensity. Thus, when the reflected light 45a moves, differences are caused in the output voltage of the respective light-receiving regions 44a. From the caused differences in the output voltage, a movement amount of the reflected light 45b can be known, from which the deflection amount θ of the cantilever 42 can be known.
A detection sensitivity S in the detection optical system 40 by the optical lever method is given by S=D/d=2L/K, where a displacement of the reflected light 45b on the light detection means 44 is D, a length of the cantilever 42 is K, a light path length of the reflected light 45b is L, and a deflection displacement of the fore-end of the cantilever 42 is d. Here, for example, if L=50 mm, K=100 μm, then S=1000 is obtained. In this manner, the use of the detection optical system 40 by the optical lever method enables very highly-sensitive displacement detection to be performed with a simple configuration.
FIG. 5 is one example of a scanning type prove microscope 50 by the optical lever method. Incident light 52 from a laser diode 51 is bent by a mirror 54 arranged immediately above the cantilever 53 to be applied to a cantilever 53. Reflected light 52a reflected at the cantilever 53 enters light detection means 55. The light detection means 55 is a quadrant photodiode.
A sample 56 is fixed on a sample holder 57, and the sample holder 57 is provided on a fine movement mechanism 58. For the fine movement mechanism 58, a piezoelectric element (piezo element) is ordinarily used. The fine movement mechanism 58 includes a fine vertical movement mechanism 59 to adjust a fine vertical movement amount of the sample 56 and a fine horizontal movement mechanism 60 to scan the sample 56 in a horizontal plane.
During the scanning of the sample 56, the fine vertical movement amount z of the sample 56 is adjusted by the fine vertical movement mechanism 59 so that the deflection amount θ of the cantilever 53 constantly has a reference value. The fine vertical movement mechanism 59 is operated by applying a voltage to the piezoelectric element. From the operation of the fine vertical movement mechanism 59, a surface shape of the sample 56 can be known. An optical microscope 61 enables the cantilever 53 and the sample 56 to be observed.
On the other hand, there is also a scanning type probe microscope having a structure in which the sample is fixed and the cantilever holding portion is finely moved vertically. This is used in the case where a large sample such as a semiconductor wafer is observed, or in the case where a plurality of cantilevers are driven independently. In this structure, a position of the reflected light on the light detection means is affected not only by the deflection amount θ of the cantilever but also the fine vertical movement amount z thereof. Thus, in the measurement output, the effects by the deflection amount θ and the fine vertical movement amount z of the cantilever are included. Accordingly, accuracy and reliability of the surface shape of the sample by the output of the fine vertical movement amount z of the cantilever holding portion has a problem.
As shown by dashed lines in FIG. 4(c), the cantilever 42 may finely move vertically (in the fine vertical movement amount z). Although the deflection amount θ of the cantilever 42 does not change, the reflected light 45a entering the light detection means 44 moves as shown by reflected light 45c indicated by a dashed line. This movement is apparently the same as that when the deflection amount θ of the cantilever 42 changes as shown in FIG. 4(b). Thus, when the reflected light 45a moves on the light detection means 44, whether this movement is due to the change in the deflection amount θ of the cantilever 42 or due to the fine vertical movement amount z cannot be distinguished.
In the case of the dynamic mode, there is a method in which an output signal of the light detection means 44 is separated into an alternate current signal and a direct current signal, so that the alternate current signal is used for the control of the distance between the probe and the sample, and the direct current signal is imaged as a displacement amount of the fine vertical movement of the cantilever holding portion 41 (e.g., JP 2004-69445 A). This enables an effect by change in relative position between the cantilever holding portion 41 and the optical system to be eliminated, and since the actual displacement amount of the cantilever holding portion 41 is imaged, an error of the surface shape by hysteresis of a fine vertical movement element can be removed.
However, as in JP 2004-69445 A, the method for separating the output signal of the light detection means into the alternate current signal and the direct current signal is effective only in the dynamic mode, but is not available in the contact mode.
There is a method in which in order to increase measurement accuracy, a laser optical system and the light detection means are finely moved vertically in conjunction with the cantilever to thereby completely compensate for the fine vertical movement of the cantilever holding portion (Atomic forcemicroscope with improved scan accuracy, scan speed and optical vision (REVIEW OF SCIENTIFIC INSTRUMENTS, VOLUME 74, NUMBER 10, OCTOBER 2003). However, in this method, a mechanism is very complicated and thus, not general.